Four main research objectives have been initiated and are proposed to be continued. Electron microscopy of individual nucleic acid molecules will be the central method used. a. Physical properties of viral nucleic acids will be studied, specifically size and shape. This will include calibration of the method of molecular weight determination and parameters controlling the shape of DNA molecules. b. Physical gene-mapping will be done on extrachromosomal DNA plasmids from bacteria carrying infectious resistance (R) factors against antibiotics, using hybridization techniques to locate genetic deletions, to identify and to map regions of homology between plasmids of different origin, in order to clarify evolutionary and medical aspects. c. Tertiary structures of purified DNA, induced by controlled dehydration, and the condensing principle of chromatin will be investigated with new techniques as an attempt to understand the structure and function of eukaryotic chromosomes. d. The virus-host relationship in cultured mammalian cells (human KB line) will be investigated by electron microscopy of transcription of adenovirus. Emphasized will be the physical location, selection and transcriptional timing of virus genes and on the role of each in the cellular response of productive or abortive infection or of transformation to uncontrolled growth.